The purpose of this study is to identify antecedents of severe and mild mental retardation in a large population of children studied from the prenatal period to age 7. The prevalence of mild retardation (1% in whites and 5% in blacks) and to a lesser degree, of severe retardation (0.5%) was inversely related to socioeconomic status. Among the severely retarded, 25% of whites and 50% of blacks had no major genetic or neurological abnormality. Perinata risk factors for severe retardation include Down's syndrome, major CNS malformations, neonatal seizures, clinical signs of perinatal hypoxia and maternal seizures in pregnancy. Among severely retarded children with unknown etiology, non-CNS malformations, peripheral nerve abnormalities, signs of hypoxia, and maternal urinary tract infection during pregnancy were important perinatal risk factors. Mild mental retardation is also associated with some perinatal complications, but less strongly than with indices of maternal intelligence, education, and socioeconomic status of the family. Physical and mental development of the retarded children are discussed as are familial patterns of mental retardation and an analysis of drugs taken during pregnancy. A monograph is in preparation.